Joyce Penner, professor in the University of Michigan Department of Atmospheric,
Oceanic and Space Sciences, U-M graduate student Yang Chen, and assistant professor
Xiquan Dong from the University of North Dakota Department of Atmospheric Science,
reported their findings in the Jan. 15 issue of the journal Nature.

Most evidence that increased levels of fossil fuel particles (aerosols) affects
the reflectivity of clouds, thereby producing a cooling effect on the climate,
has been indirect. “This made it difficult to determine the impact this phenomena,
known as the indirect aerosol effect, has on the global climate,” Penner said. “Our
data makes the direct connection and opens new areas of study.”

Solar radiation, which adds to global warming, is reflected back into space
by clouds. Cloud droplets are increased with higher levels of aerosols, allowing
for less radiation, or heat, to reach the lower atmosphere. The end result
is a measurable cooling effect on the climate.

Using atmospheric data gathered from a site in Oklahoma, a typical continental
site with a high concentration of aerosols, and a typical Arctic site in Barrow
Alaska with low aerosol concentration, the researchers were able to show that
the difference in cloud reflectivity at the two sites was caused by the difference
in aerosol levels. The researchers also provided important evidence that the
computer simulation model used in the study was capable of estimating cloud
optical properties determined over a broad range of aerosol concentrations.

“This study is important for two reasons,” Penner said. “First, it provides
evidence that there is some cooling of the climate due to anthropogenic aerosols.
Second, the simulation model we used has been shown to be a valuable tool in
determining more directly the impact of aerosols on the climate.”

Penner cautioned that over longer time scales in the future, the climate cooling
due to the indirect aerosol effect will be minimal when compared to the climate
warming of carbon dioxide. “We've shown that there's more work to be done to
discover all of the various ways we affect the climate.”